The present invention relates to a measuring arrangement for determining the gaseous radioiodine concentration in a carrier gas wherein the radioiodine is separated in a sorption agent and accumulated and the radioiodine activity is detected by means of a detector.
In nuclear plants, particlarly in nuclear energy plants, it is important to measure the emission of gaseous radioactive iodine isotopes, particularly .sup.131 I.
Since the concentration of radioiodine in the air of the plant and particularly at its chimney is extremely low, a relatively large amount of exhaust air must be conducted through a suitable adsorber, e.g. catalyst material impregnated with AgNO.sub.3, to obtain measurable activity by way of accumulation at the sorption agent. In the meantime, highly sensitive measuring instruments have become available for this purpose. In principle, these instruments are comprised of an adsorber cartridge through which passes a large volume stream, and a detector for gamma quanta, e.g. a scintillation detector employing an NaI/Tl crystal or a semiconductor detector, such as, for example, Ge(Li) as disclosed in German Pat. No. 2,251,189. The aim of these measuring devices is the realization of the highest possible detection sensitivity so as to be able to accurately measure the extremely small amounts of iodine released through the chimney.
However, the dynamic range of such highly sensitive measuring devices is limited. In case of serious malfunction, iodine concentrations of such magnitude could occur that the measuring range would be insufficient. It thus initially appears to be obvious to measure very high iodine concentrations in the air directly. However, the latter is not possible since in addition to the radioiodine, radioactive noble gases exist simultaneously in the air and their concentration is orders of magnitude higher than that of the radioiodine so that they would completely overdrive the detector. Therefore, accumulation on an adsorber is necessary even with very high radioiodine concentrations so as to be able to raise the iodine concentration component with respect to the noble gas concentration. The sorption agent is here advantageously accommodated in easily replaceable cartridges.
An arrangement is known, for example, which operates with a turntable in which a large number of sorption cartridges are accommodated. If the activity of the radioiodine collected in one cartridge becomes too high for the detector, the turntable is caused to undergo a rotation step and makes available a fresh sorption cartridge.
In another arrangement, operating according to a similar principle, the sorption agent is disposed between the sides of a stepwise movably toothed wheel.
Both arrangements have in common the stepwise switching from one sorption cartridge or chamber to the next one so that continuous operation is impossible. Moreover, both arrangements require considerable engineering expenditures for secure sealing arrangements.